1. Field of the Invention
The present invention relates to an amplifying circuit used in audio systems.
2. Description of the Related Art
FIG. 1 shows a conventional bridge-connected audio amplifying circuit. The amplifying circuit includes first and second operational amplifiers 11, 12, connected as inverting amplifiers. The inverting input (xe2x88x92) of amplifier 11 is connected to an input terminal E of the system via a resistor 13 and a coupling capacitor 14 connected in series. Output O1 of amplifier 11 is connected to the inverting input (xe2x88x92) via a resistor 15. The inverting input (xe2x88x92) of amplifier 12 is connected to output O1 of amplifier 11 via a resistor 16 and to its output O2 via a resistor 17. Outputs O1 and O2 of amplifiers 11, 12 are connected across a load 18, typically a loudspeaker able to give out sounds according to the current flowing therethrough. The non-inverting inputs (+) of amplifiers 11, 12 are connected together to a node BP of a resistive divider including a resistor 19 connected between node BP and a supply terminal VCC, and a resistor 20 connected between node BP and ground GND. A capacitor 21 is connected in parallel with resistor 19. Capacitor 21 has the function of filtering the noise generated by resistors 19 and 20 and for absorbing possible variations of the voltage at supply terminal VCC.
The gain of amplifier 11 is given by the ratio of resistances 15 and 13. The gain of second amplifier 12 is generally chosen to be equal to xe2x88x921 by setting an identical value for both resistances 16 and 17.
The expression of voltage VCH across load 18 is given by the following equation:
VCH=VO1xe2x88x92VO2=xe2x88x922(R15/R13)*(VMxe2x88x92VBP) 
where R13 and R15 are the respective values of resistances 13 and 15; and VO1, VO2, VBP, and VM are the voltages at outputs O1 and O2 of amplifiers 11, 12, at node BP, and at a node M between capacitor 14 and resistor 13, respectively.
The divider formed of resistors 19 and 20 sets the voltage at node BP to a reference voltage. For example, the reference voltage may be chosen to be equal to VCC/2 and the values of resistances 19, 20 are then set to a same value. In normal operation, in the absence of a signal at input terminal E, voltages VM and VBP are equal to the reference voltage and the voltage across the load is zero. When a voltage is applied to input terminal E, voltage VM is equal to the reference voltage plus the variable component of the input voltage, coupling capacitor 14 suppressing the D.C. component of the input voltage.
Accordingly, the voltage across load VCH is equal to the variable component of the input voltage multiplied by amplification gain xe2x88x922R15/R13. By choosing an adapted ratio of the values of resistances 15 and 13, the peak-to-peak load voltage can be significantly amplified.
FIG. 2 shows a circuit similar to that of FIG. 1 further including a stand-by system 25 having the function of maintaining supply voltage VCC which may be used by stages upstream of the amplifying state, while reducing the specific consumption of the amplifying portion. Stand-by system 25 receives a control signal and is connected at a first output to inhibition terminals A1 and A2 of amplifiers 11 and 12. An output of stand-by system 25 is further connected to a transistor 26 in series with resistor 19. Upon reception of the specific control signal, stand-by system 25 also blocks transistor 26 to suppress the consumption of resistors 19, 20.
FIG. 3 shows the evolution of voltages along time at given points of the amplifying circuit of FIG. 1 at the device power-on, that is, when the supply voltage passes from 0 to voltage VCC. Curve VALIM shows the variation of the supply voltage along time. Curve VM shows the variation of the voltage at node M along time, curve VBP showing the variation of the voltage at node BP along time.
At the circuit power-on, supply voltage VALIM almost instantaneously switches from 0 volt to VCC. The voltage at node BP settles at the reference voltage. The voltage at node VM also settles at the reference voltage, for example, VCC/2. It should be noted that curve VM reaches an equilibrium level in a time shorter than that of curve VBP.
The rise time of the voltage at node BP is mainly determined by the values of capacitance 21 and of resistances 19, 20. It generally is on the order of from 50 to 150 ms. It is generally not possible to guarantee an identical time constant xe2x80x9cseenxe2x80x9d by node M, which implies different rise times for the voltages at nodes M and BP.
On FIG. 4, curve VMxe2x88x92VBP shows the difference between the voltages at nodes M and BP and curve VCH1 shows the voltage applied across load 18 for the circuit of FIG. 1.
At the circuit power-on, the operational amplifiers supplied by the supply voltage being almost xe2x80x9cinstantaneouslyxe2x80x9d on, the difference between the voltages at nodes M and BP is reflected on load 18, multiplied by the amplifying gain. The voltage applied to the load, due to a high amplifying gain, is often sufficient to cause a characteristic audible and unpleasant noise.
In the case where the amplifier is equipped with a stand-by unit 25, the problem is also posed upon switching from the stand-by state to a normal operation state, since in this switching, the voltage at node BP will settle to the reference voltage while the voltage at node M has already settled to the reference voltage.
Upon switching from a normal operating state to a stand-by state or from a normal operation state to an off state, the voltages at nodes M and BP switch from the reference level to a zero voltage in different durations, for the same reasons as mentioned previously. Generally, the supply of amplifiers 11, 12 being xe2x80x9calmost-instantaneouslyxe2x80x9d interrupted, said amplifiers no longer amplify directly across the load the difference between the voltages at nodes M and BP. However, amplifiers 11, 12 may stay on, so that a current generated by the difference between the voltages at nodes M and BP could cross the load. However, this current often is very small and only rarely translates as a characteristic audible and unpleasant noise of the loudspeaker. In the case where the switching of the supply voltage from VCC to a zero voltage exhibits a non-negligible time constant, amplifiers 11, 12 and comparator 30 may remain supplied for some time. In this case, the voltage difference at nodes BP and M remains amplified across the load and may be the cause of a characteristic audible and unpleasant noise.
The disclosed embodiment of the present invention provides an audio amplifying circuit that includes a system for reducing unwanted noise appearing at the circuit turning-on from an off state or a stand-by state.
Accordingly, the embodiment of the present invention provides an amplifying circuit having an amplifier circuit that receives an input voltage and a reference voltage circuit to generate a reference voltage equal to a fraction of the circuit supply voltage, a time constant circuit for generating a voltage, the amplifying circuit including means for, upon power-on, inhibiting the amplifier circuit for as long as the difference between the value of the provided reference voltage and the voltage at the output of the time constant circuit is greater than a determined threshold.
According to another feature of the present invention the amplifying circuit includes a first amplifier having a first amplifying gain, receiving at a first input the input voltage, and a second amplifier having a second amplifying gain, and receiving at a first input the output of the first amplifier, the outputs of the first and second amplifiers being connected to a load, second inputs of the first and second amplifiers being connected together to a node receiving the reference voltage.
According to another feature of the present invention, the time constant circuit includes a first resistive divider including a first resistor connected between the node and a high supply terminal and a second resistor connected between the node and a low supply terminal, a capacitive element being connected in parallel across one of the first or second resistors.
According to another feature of the present invention, the means for inhibiting the amplifying circuit include a comparator receiving at a first input the provided reference voltage and connected at a second input to the node, and the output of which controls the setting to high impedance of the first amplifier or of the second amplifier for as long as the difference between the reference voltage and the node voltage is greater than a determined threshold.
According to another feature of the present invention, the means for inhibiting the amplifier circuit include a second resistive divider connected in parallel on the first voltage divider and including third and fourth resistors in series, the first input of the comparator being connected to a node between the third and fourth resistors.